Conventional four stroke engine combustion cycles, also known as Otto Cycle engines, comprise four distinct strokes: the intake, compression, power, and exhaust strokes. During all four strokes, a piston in a cylinder of the four stroke engine moves between a top dead center (TDC) position, and a bottom dead center (BDC). As the piston moves towards the BDC position, the volume formed between the piston dome and the combustion chamber increases, and vice versa as the piston moves towards TDC. During the intake stroke and the power stroke, the piston moves from the TDC position to the BDC position, whereas during the compression stroke and the exhaust stroke, the piston moves from the BDC position to the TDC position. However, during the intake stroke an intake valve is open and an exhaust valve is closed. In both the compression stroke and the power stroke, the intake valve and the exhaust valve are typically kept closed. During the exhaust stroke, the intake valve is closed, and the exhaust valve is opened.
Thus, in conventional four stroke engines, the intake valve is opened during the intake stroke, and gasses enter the combustion chamber due the negative pressure created as the piston translates towards the BDC position. Additionally, fuel may be injected directly in to the combustion chamber by a fuel injector during the intake stroke. Then, once the piston reaches the BDC position and begins to translate back towards the TDC position at the start of the compression stroke, the intake valve is closed. A spark plug ignites the air/fuel mixture in the combustion chamber before the piston reaches the TDC position, and due to the ignition from the spark plug, the air/fuel mixture expands as the piston reaches the TDC position and begins to move back towards the BDC position. Thus, during the power stroke that follows, the ignited air/fuel mixture exerts a force on the piston which drives the piston towards the BDC position. It is during the power stroke therefore, that power is generated by the engine. Finally, during the exhaust stroke, the exhaust valve opens, allowing the air/fuel mixture to exit the combustion chamber.
However, in an effort to increase the thermodynamic efficiency of the engine, many combustion engines employ a combustion cycle referred to as the “Miller” Cycle. Unlike the conventional four stroke combustion cycle described above, in a Miller Cycle engine, the intake valve is maintained in an open position during a first portion of the compression stroke. As the piston initially moves towards the TDC position in the compression stroke, a portion of the air/fuel mixture, and recirculated exhaust gases under certain conditions such as high engine load operation, may be expelled back into an intake port towards an intake manifold of the engine through the still-open intake valve. This movement of a portion of the air/fuel mixture and exhaust gases in the combustion chamber back towards the intake manifold may be referred to herein as a reversion event. The efficiency of a Miller Cycle engine is increased by the use of a boosting system, either a supercharger or a turbocharger. Additionally, such engine systems may include a charge air cooler for cooling the intake air.
Because the air/fuel mixture may only be compressed after the intake valve is closed, the air/fuel mixture may only be compressed for approximately the final 25% of the compression stroke. In the conventional Otto Cycle engines, the compression ratio is limited due to self-ignition (e.g., detonation) of the compressed, and therefore hot, air/fuel mixture. However, due to the reduced compression of the air/fuel mixture during the compression stroke in a Miller Cycle engine, the compression ratio of a Miller Cycle engine may be increased relative to conventional Otto Cycle engines. Therefore the efficiency of the Miller Cycle engine may be increased relative to Otto Cycle engines.
However, the inventors herein have recognized potential issues with such Miller Cycle engine systems. As one example, the air/fuel mixture in the combustion chamber may be at a higher temperature than gasses in the intake port and an intake manifold due to residual heat in the combustion chamber from previous combustion cycles. Specifically, the heat produced during each combustion cycle may not be fully dissipated at the end of each combustion cycle, resulting in the combustion chamber being at a higher temperature than the intake port and intake manifold. When the air/fuel mixture and recirculated exhaust gases revert back towards the intake manifold through the open intake valve during the first portion of the compression stroke, the hotter air/fuel mixture may reduce the effectiveness of a charge air cooler in cooling an incoming air charge and reverted exhaust gases may leave particulate matter in the air cooler. Further, continued exposure to the hot air/fuel mixture and recirculated exhaust gases may lead to charge air cooler degradation. In both cases, the cooling efficiency of the incoming air charge to the combustion chamber during the intake stroke may be reduced. The reduced cooling efficiency of the charge air cooler may result in increased intake air temperatures, which may cause unintentional detonation, also known as engine knock. Further, crankcase oil may collect in the substrate of the charge air cooler, which may dilute the air/fuel mixture, thereby lowering the octane of the fuel. Lowering the octane of the fuel may also contribute to unintentional detonation events.
Additionally, the portion of the air/fuel mixture that flows out of the combustion chamber to the intake port during the reversion event, opposes the direction of flow of gasses in the intake port during the intake stroke. As such, it may take time for the flow direction in the intake port to be reversed before the next intake stroke. As a result, there may be delay in the flow of gasses into the combustion chamber and therefore the torque delivered by the engine, upon the initiation of an intake stroke.
In one example, the issues described above may be addressed by a method comprising positioning an intake valve, coupled to a cylinder of a four-cycle internal combustion engine, in an open position during a portion of an intake stroke through a portion of a compression stroke of a piston reciprocating within said cylinder, supplying air to said intake valve from a first source; and, injecting air against said intake valve from a second source while said intake valve is open during said compression stroke. In this way, the amount of an air/fuel mixture flowing out of a combustion chamber through the intake valve during a portion of the intake stroke may be reduced.
In another representation, the issues described above may be addressed by a method comprising opening an intake valve coupled to a cylinder of a four-cycle internal combustion engine during an intake stroke of a piston positioned in said cylinder, said engine including an intake manifold coupled to said intake valve through an intake port, supplying air from said intake manifold through said intake port to said intake valve, recirculating a portion of exhaust gases from said engine into said intake valve, closing said intake valve during a compression stroke of said piston, reverting a portion of air and said recirculated exhaust gases from said cylinder through said intake valve and said intake port during said compression stroke while said intake valve is open, and injecting air from an air accumulator into said intake port toward said intake valve against said reverted air and exhaust gases while said intake valve is open during said compression stroke.
As one example, the method may additionally or alternatively comprise supplying compressed air through a heat exchanger to cool said compressed air and routing said cooled compressed air to said intake manifold.
In another example, the method may additionally or alternatively comprise controlling the timing and duration of the air injected into the intake port to either reduce or substantially stop said reverted air and said recirculated exhaust gases from entering said heat exchanger.
In another representation, an engine system may comprise: an air injector positioned in an intake port upstream of an engine cylinder and downstream of a compressor and charge air cooler, an air accumulator fluidly coupled to the air injector for providing compressed air thereto, and a controller with computer readable instructions. The computer readable instructions may include instructions for injecting a desired amount of compressed air from the air accumulator to the intake port via the air injector when the engine cylinder is in a first portion of a compression stroke, where the first portion of the compression stroke is a portion of the compression stroke in which an intake valve of the engine cylinder is in an open position, such that gasses flow between the cylinder and the intake port and otherwise not injecting air to the intake port from the air accumulator.
In this way, the amount of an air/fuel mixture reverted to an intake port through an open intake valve during a portion of a compression stroke may be reduced. By reducing the amount of the air/fuel mixture reverted through the intake valve, degradation to a charge air cooler may be reduced. Further an amount of mixing and atomization of the air/fuel mixture may be achieved by injecting compressed air into the intake port. Specifically, the air injected into the intake port may oppose the direction of flow of the air/fuel mixture from the combustion chamber enters the intake port. As a result, the mixing and atomization of the air/fuel mixture may be increased. Further, the injecting of the air may reverse the direction of motion of the reverted air/fuel mixture, and may therefore reduce its momentum. In this way the responsiveness of an engine may be increased. By reducing the momentum of the air/fuel mixture to the intake port during the compression stroke gasses may flow into the combustion chamber more quickly during a subsequent intake stroke.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.